HANDBOOK OF INTERCOM SYSTEMS ENGINEERING FIRST EDITION

Transcription

1 HANDBOOK OF INTERCOM SYSTEMS ENGINEERING FIRST EDITION Preliminary Rev. 4, 3/2002

2 The Fine Print The Handbook of Intercom Systems Engineering, first edition, Copyright 2000 by Telex Communications, Inc. All rights reserved. Printed in the United States of America. Except as permitted under the United States Copyright Act of 1976, no part of this publication may be reproduced or distributed in any form or by any means, electronic or mechanical, including photocopying and recording, or by any information storage or retrieval system without the prior written permission of Telex Communications, Inc., unless such copying is expressly permitted by federal copyright law. Address copying inquires to: Telex Communications, Inc. Attn: VP, Intercom Products Portland Ave S Burnsville, Minnesota USA Information contained in this work has been created or obtained by Telex Communications Inc. from sources believed to be reliable. However, Telex Communications, Inc. does not guarantee the accuracy or completeness of the information published herein nor shall Telex Communications Inc. be liable for any errors, omissions, or damages arising from the use of this information. Telex Communications, Inc. is not attempting to provide professional services through the publication of this book, but rather intends only to provide information. If such professional services are necessary or desired, users of this book should seek such professional assistance.

3 TABLE OF CONTENTS Preface About the Authors Intercoms An Overview Introduction Party-Line Systems Matrix Systems Wireless Systems Accessories Before We Begin IFB ISO Tally The Rest Of The Book Introduction to Party-Line Intercom Systems Introduction Some Definitions Party-Line (PL) systems / Conference Line Intercom Systems Two-Wire Balanced Line Full Duplex Decibel (db) Beltpack Biscuit Main Station Master Station Sidetone Crosstalk A Short History Present Day Systems and Manufacturers System Components and Their Function Belt Pack Headset User Station Functional Description Speaker User Station Functional Description Master Stations Some Technical Notes About The Stations Above How Each System Works System Powering Headset User Stations Speaker User Stations Master Stations Cabling Outstanding Features of Each System Call Lights Limitations of Each System Summary (Some Definitions) (A Short History) (Present Day Systems and Manufacturers) (System Components and Their Function) (How Each System Works) (Outstanding Features of Each System) i

4 (Limitations of Each System) Design of Party-Line Intercom Systems Overview Defining And Meeting Your Needs Application 1 Generic Single Channel Systems Audiocom Party-Line Intercom Equipment Listing # Clear-Com Party-Line Intercom Equipment Listing # RTS TW Party-Line Intercom Equipment Listing # Application 2 Two-Channel System: TV, School, Cable Audiocom Party-Line Equipment Listing # Clear-Com Party-Line Equipment Listing # RTS TW Party-Line Equipment Listing # Application 3 Theater System Audiocom Party-Line Equipment Listing # Clear-Com Party-Line Equipment Listing # RTS TW Party-Line Equipment Listing # Application 4 Training Systems Audiocom Clear-Com RTS TW Application 5 Medium System for Television The IFB System (One Way Communications System) How an IFB Works Studio and Some Field Applications Field Application, Sports Field Application, ENG (Electronic News Gathering) Connecting (Interfacing) to Other Communications Systems A Typical Interfacing Problem Interfacing Issues Modes Level Problems Signal / Data Conversion Call Light Data Interfacing Practice Interfacing Television Camera Intercom Systems to TW Systems General Camera Configuration Information for Television Cameras (except ENG units) The Problems in Interfacing to Cameras Alternatives for Interfacing to Television Cameras Some Practical Considerations Headset Cable Lengths Headphone Impedances Wiring Practices/Workmanship Standards Unbalanced vs. Balanced Extended Range On Part Or All Of The System Cable Considerations Crosstalk Crosstalk Through A Common Circuit Ground Crosstalk Through A Mutual Capacitance Of Two Conductors A Low Crosstalk Approach To Interconnection Distances/Conductor Sizes/Distributed vs. Central Connection System Current/System Capacitances/Loading Temperature Range Consideration Cooling Requirements Moisture / Contamination Protection Magnetic Fields: Hum Problems SUMMARY (Defining and Meeting Your Needs) (The IFB System (One Way Communications System)) ii

5 (Connecting (Interfacing to Other Communications Systems)) (Some Practical Considerations) Introduction to Matrix Intercom Systems Introduction Definitions History of Matrix Intercoms Modern Day Matrix Intercoms Special Considerations Advantages Size Configurability Types of Communications Supported Ancillary Functions Basic Ancillary Functions via GPI/O General Purpose Input / Output More Complex Ancillary Functions Disadvantages Size Cost Complexity Design of Matrix Intercom Systems Introduction Back-to-Basics RTS Matrix Intercom Systems To Begin Let s get started Studio A Floor Control Room Other Cable Considerations Audio and Data Considerations Polling Issues Very Large Systems, Split Operation and Trunking Interfacing Signal Formats Interconnecting Matrix, PL, and Wireless Systems Software Considerations Introduction to Wireless Intercom Systems Introduction to Wireless Intercoms History of Wireless Intercoms Modern Day Wireless Intercoms Special Considerations Design of Wireless Intercom Systems Introduction Back-to-Basics Interference Transmitters and Receivers Antenna & Cable Considerations Installation iii

6 Determining Intercom Needs Conference Versus Point-to-Point Requirements Fixed vs. Mobile Requirements A General Overview Determining Intercom Needs, two-wire, four-wire, or both? Small Studio or ENG Vehicle MCE325 Modular Programmable Station PS15 Power Supply/MCP2 Rack Kit TW5W Splitter IFB325 Talent User Station BP325 Programmable Belt Pack Headsets and Earsets (not shown) Medium Sized Studio and Mobile Intercom Two-wire Case (Medium Intercom) G1G5 Master Station System Interconnect PS31 Power Supply SAP1626 Source Assign Panel BOP220 Connector Translation Assembly IFB Central Electronics Unit A Splitter Talent User Station MCE325-K Programmable User Station BP319 Belt Pack BP325 Programmable Belt Pack Telos Link Headsets and Earsets (not shown) Four-wire Case (Medium Intercom) Zeus DSP2400 Matrix KP96-7 Keypanel TIF-2000 Intelligent Telco Interface MKP4-K Modular Keypanel IFB828 IFB Power Supply SSA324 System-to-System Adapter PS15 Power Supply SAP612 Source Assign Panel MRT327-K Modular User Station PAP951 Program Assign Panel and UIO256 GPI Cameras in the Medium Intercom Large Studio or Mobile Vehicle Determining the Makeup of the Intercom Matrix First Step--Determine the Size Users IFB Circuits Cameras Miscellaneous Static Party-Lines Wireless Intercom Telephones Studio Announce and Dressing Room Paging Second Step--Determine the Panels KP96-7 Keypanel KP96-6 Keypanel Other Considerations in Determining Intercom Needs Physical Constraints two-wire Conference Systems Four-Wire Point-to Point Systems How old is Too Old? Expandability Interoperability Maintenance Budget iv

7 Glossary Index v

8 vi

9 LIST OF FIGURES Simple Party-Line System Simple Matrix System Wireless Intercom Examples Example of Interfacing a TW System to a Matrix System Complex Matrix Intercom System Audiocom intercom concept Clear-Com intercom concept RTS TW intercom concept RTS TW user station block diagram Generic single channel Audiocom system Generic single channel Clear-Com system Generic single channel RTS TW system Small TV operation Theater application Audiocom based training intercom system Clear-Com based training intercom system RTS TW based training intercom system Medium intercom system for television The KP-32 is a good example of an advanced user station (keypanel) Example of Matrix Ports A Comparison 3x3 vs. 9x9 Matrices A comparison of the 9400 Intercom System to the 9500 Intercom System (see inset). The 9500 represented a tremendous reduction in physical size An example of how multiple signals are time-sliced for use in a TDM system Conventional Matrix vs. TDM Matrix Typical Keypanel Simplified Low-Cost User Station Use of Source Assignment Panels such as this SAP-1626 allow the rapid reconfiguration of PL systems without changing any cables Typical ADAM Matrix Connections A wide variety of keypanel options exist. Here we have a selection of RTS keypanels that fit a range of needs. Small keypanels such as the (A) KP-12LK and (B) WKP-4 provide an interface for those with limited keypanel needs. The (G) KP-96-7, a medium sized unit, was the workhorse of the RTS keypanel line until the 1980 s and 1990 s. The (C) KP-32 is the top of the line keypanel, and can be enhanced through additional options, such as the (D) EKP-32 expansion panel, and the (F) LCP-32/16 level control panel. The (E) KP-8T is an example of a specialty keypanel that makes use of an empty bay in a Tektronix vectorscope ADAM (including ADAM CS and Zeus ) Intercom Cable Connections

10 A Comparison of Relative System Sizes Separate Studios, Separate Intercom Fixed Trunking Intelligent Trunking Cascaded Trunking TW and Matrix Signal Flows Wireless Intercom Interfaced to Matrix Intercom GPI/O Implemented PTT (Push-To-Talk) TW to Matrix Interface 81 ADAM and ADAM CS Basic Components Matrix Intercom Remote Control The first beltpack based wireless intercom system An example of a modern day wireless intercom system The RadioCom BTR-800 System is an outstanding example of the next generation of wireless intercom systems NTSC channel configuration The E an H fields exist in two separate planes, 90 apart from each other An example of wireless transmission and reception An example of electromagnetic waves being radiated An example of reflected RF waves The orientation of the radiator (antenna) determines the polarization, and therefore, the orientation of the E and H fields Waves that are in phase combine to form a larger wave Waves that are out of phase cancel each other An example of combining waves that are not 180 out of phase An example of multipath in its most basic form Transmitter block diagram Receiver block diagram Good linearity is a must for faithful signal reproduction A comparison of the radiation patterns for an Isotropic Radiator (theoretical) vs. a Dipole (practical) An example of a Yagi antenna Telex s ALP-450 is an example of a Log Periodic antenna The typical parts of coaxial cable Wiring differences between larger conference and point-to-point styles Figure 3. Block diagram of a medium sized intercom system using two-wire. The forms of communications depicted here are six conference lines and eight IFB circuits Block diagram of a medium sized intercom system using the Zeus four-wire matrix. The forms of communications depicted have increased to include point-to-point and ISO Figure 5. Block diagram of a large size intercom system using a twin ADAM configured as a 200x200 matrix

11 CHAPTER 0 CHAPTER 0 CHAPTER 0 CHAPTER 0PREFACE Welcome to the Telex Communications, Inc. Handbook of Intercom Systems Engineering. The idea for this book came, as it does with many books and inventions, over drinks at a bar. A few of us intercom types were discussing our varied histories and experiences. We added up the years each of us had in the intercom system industry and between the four of us we hit the 75 year mark. Add the rest of the gang at Telex into that estimate and we are well past the century mark, quickly closing in on the two century mark. It was then we decided that we were getting old and had spent too much time dealing with intercoms. Someone commented that it was a shame that the younger generation didn t really know what we seasoned pros did and suggested that we should pass down our profound body of knowledge for the good of intercom-kind. The idea for the book sort of hibernated for a bit after that (as did we). Weeks later we found ourselves planning for a trade show and discussing the appropriate swag for giveaways. After some discussion, we decided a well written, reasonably impartial, complete reference / tutorial on intercom system design would be a great thing useful, desirable, business related, and maybe something inspirational. We hope those that read this book take advantage of the knowledge they can glean from it and expand the capabilities of their own intercom systems. And, maybe they will use some intercom equipment from Telex. In the process, we may go down in history as the guys who wrote the book on intercoms. The book you are starting has a number of goals; it is intended to be a systematic tutorial for the novice user and an encyclopedic reference for the designer in the midst of a project. It is NOT a 100+ page sales brochure for Telex products. Rather, it is a resource intended to take the reader through the different types of intercom systems and needs, compare them, point out strengths and weaknesses, and provide many real-life examples of working systems. This book will be updated regularly to keep pace with changes in technology. On the enclosed CD you will find a good deal of technical information, systems examples, and some marketing fluff such as Telex product sheets, catalogs, operating manuals, etc. Throughout the book, we have strived to provide real examples with real products. Many of the examples will make use of Telex products, as that is what we know best Telex AudioCom, RTS Matrix, RTS TW, RadioCom Wireless and Telex Headsets. If we get to a point with an example where the equipment needed or best suited is not one of our products, we will tell you what that product is and how to find it. I have often joked that intercoms are the stepchild of the industry no one (or VERY, VERY FEW) people decided in high school what they wanted to do with their lives is be Mr. (or Ms.) Intercom. People tend to get dragged kicking and screaming into dealing with the design, installation and support of intercom systems because they were in the wrong place at the wrong time. What they later learn is that they have developed a valuable bit of niche expertise that can be in great demand. The one goal above all with this book is to provide a solid body of work, in a useful form to all those who have been, and will be, dealing with specification and operation, as well as, design, 3

12 installation and support of intercom systems. In other words, we hope this book helps you get the absolute most out of your communications systems. Apart from the story of the bar and the trade show, there is another serious reason why we have written this book. Intercoms (in our opinion) are a neglected, underrated, taken for granted part of the technical world they are not glamorous nor interesting. I have at times made the comment that intercom systems have a lot in common with toilets (no off color jokes to follow). They are often the last system designed into an environment, they are often cheaply done, they are PRESUMED to be always available and always working, and when they are NOT it QUICKLY becomes a crisis and the plumber, all of a sudden is worth ANY AMOUNT OF MONEY to return the toilet to its normal functioning condition, FAST! Now, let s take the same scenario except in the intercom world. Consider a live television show, a camera fails, or a microphone fails, and the audio operator can t hear the guest, or a tape jams in a VTR. No problem, we ll just TELL the TD to take another camera, and TELL Camera 2 to change its shot, or the audio operator will ASK the stage manager to get a spare microphone to the talent, or the director will TELL the talent to ad-lib until the tape can be salvaged. WHAT DO YOU MEAN, NO ONE CAN COMMUNICATE THESE SIMPLE INSTRUCTIONS!?!? Get the PLUMBER (oops INTERCOM EXPERT) NOW!!!! The intercom system, whether in a television station, on the sidelines of a football game, or in a factory is critical, and must be seamless, reliable, and work without fault to allow all needed communications to take place. This book is intended to help make that happen. We d love to know if you think we have succeeded, or failed, or fallen short with this effort, so that, as with all things in life, we can learn, grow and improve. Please send your comments to intercoms@telex.com. Ralph K. Strader Vice President & General Manager Intercom Products Telex Communications, Inc. January Handbook of Intercom Systems Engineering

13 CHAPTER 0 CHAPTER 0 CHAPTER 0 0ABOUT THE AUTHORS CHAPTER This handbook is the work of a number of past and present Telex employees, as well as, some outside experts (such as Stan Hubler). Among the contributors (in alphabetical order) are: Talal Aly-Youssef, Gene Behrend, Larry Benedict (contributor and editor), Rick Fisher, Stan Hubler, John King, Murray Porteous, Dave Richardson, Ralph Strader, and Tom Turkington. The credits for each chapter reflect the contribution of the primary author for that chapter. Through a group effort such as this, the words may actually be those of a number of individuals in any given chapter. Many other individuals have directly or indirectly contributed to this book, and not all of them can be recognized here. Many of the illustrations were prepared by John Yerxa, and many of the systems examples came from the work of Shawn Anderson, Chuck Roberts, Gene Behrend, and Geoff Rogers. 5

14 6 Handbook of Intercom Systems Engineering

15 C HAPTER 1 1INTERCOMS AN OVERVIEW CHAPTER RALPH STRADER Introduction Intercom systems, by definition, may be comprised of many different types of intercoms and subsystems. The basic building blocks can be categorized into four basic types or elements: Party-Line Systems, Matrix Systems, Wireless Systems, and Accessories. Party-Line Systems Wired Party-Line systems are systems in which a number of participants are all involved in the same conversation. Think of the telephone extensions in your home, if each person in your family picks up a telephone in your home, you will all be able to hear each other. You can talk to one another simultaneously and the person on the other end of the line will be a full participant in one public conversation. Depending on where in the world you are from, (presuming English language), you may also refer to this type of system as PL (for party-line), TW or Two-Wire from the telephone systems, where on two wires, a full duplex conversation takes place, or conference denoting the type of activity taking place in the conversation. Figure 1.1 Simple Party-Line System Note, the physical configuration and implementation of that PL or TW does not necessarily need to be on two physical wires, in most cases it is not. The specific topologies will be addressed in the chapters that follow. Chapter 1 - Intercoms An Overview 1

16 Matrix Systems Wired Matrix systems are systems in which a large number of individuals have the ability to establish private individual conversations from point A to point B. Again, going back to the telephone system in your neighborhood, you, your next door neighbor, the pizza joint down the street and the local gas station are all connected to the same central office by wires from each location back to the telephone company. At any time, you can be talking to the gas station, while your neighbor is ordering a pizza. The pizza guy does not hear you ask the mechanic about the repairs on your SUV. Depending on where in the English speaking world you are, you may refer to these types of systems as Matrix systems, crosspoint intercoms, point-to-point systems, private lines (sometimes, confusingly referred to as PL ), or by some of the brand names used: McCurdy, ADAM, Zeus, and others. Figure 1.2 Simple Matrix System MATRIX You Neighbor Pizza Joint Gas Station X X X X Like the telephone system, matrix systems have other functions and capabilities. Conferences, call waiting, busy signals, and other features are common to many matrix intercoms. They are not limited to simple point-to-point communications. Some systems even allow inter-matrix routing of signals, similar to long distance telephones calls using trunks between central offices. Having a matrix system with a number of conferences configured within it (virtual PLs) is very common. Wireless Systems You Neighbor Pizza Joint Gas Station Wireless Intercoms encompass all sorts of systems from the most basic pair of walkie talkies to cell phones to dedicated professional full duplex intercom products. The most basic feature of wireless intercoms is that they are not tethered by wires. (Didn t think this was going to be quite that basic, did you?) Seriously, wireless intercom systems are employed where the limitation of wireless systems which can include fidelity, interference, lack of range, lack of security (real or perceived), and battery life limitations are outweighed by the freedom of being cordless. This freedom can be essential in many applications try dragging a wired intercom cable into the containment vessel of a nuclear reactor. Wireless intercom systems can be designed, installed, configured and operated in PL or matrix configurations, and may very likely be connected to a hard-wired PL or matrix 2 Handbook of Intercom Systems Engineering

17 Headset Power Ext Intercom Aux Audio Portable Transmit On Portable Station Connect Volume BTR-300 Headset Controls Talk Gain O/M Push Twice to Latch intercom system at some point. They can range from as simple as a single pair of units talking to one another, to a system in which 24 or more different portable units are dynamically switched between conversations. Figure 1.3 Wireless Intercom Examples BTR-300 Transmit to Beltpacks RadioCom Transmit to Base TR-300 TR-300 TR-300 TR-300 Mirror Image Pair Telex TR-500 Base Station with 4 Remotes Wireless systems will vary tremendously worldwide, due to varying governmental radio regulations. What is common in America may be illegal in Japan, and may be unsuitable, for other reasons, in Germany. These units may be referred to by any of the types mentioned above, but, again, the unifying feature is the freedom from a wire. Accessories The fourth and final category is accessories. We are giving accessories its own separate category because of its importance. This book is addressing intercom systems. In all likelihood, many of the systems you encounter will be an amalgam of the three types mentioned above. Without accessories you cannot have a system, just a bunch of equipment. To connect a TW system to a matrix system, a converter is required to change the combined talk and listen signal from the TW to separate talk and listen signals for the matrix a hybrid provides this conversion. Chapter 1 - Intercoms An Overview 3

18 Figure 1.4 Example of Interfacing a TW System to a Matrix System To connect a matrix intercom system to a Two-way radio system, a contact closure may be required to activate the radio transmitter. A GPI (General Purpose Interface) between the matrix and the base station of the radio can solve this problem easily. To do intelligent trunking between matrix systems, across campus or across the country, the audio and control signals between the matrices could be transported over fixed pairs of wires. Realistically, however, installing a set of wires between Omaha and Los Angeles may be out of your budget so an interface allowing the use of dial-up telephone lines may be needed. Other possibilities include muxes and demuxes to allow the audio and data to be carried over an existing corporate Wide Area Network (WAN), or piggybacked as subcarriers on an existing satellite feed. 4 Handbook of Intercom Systems Engineering

19 DIR PROD RA PL01 IFB4 TD DIR NUM PL 1 2 SUST IFB LISTEN PHONE AD NEWS T1 TEL1 TEL2 FLOR CHYR ISO1 ISO2 AUD1 4 5 RELAY COPY E-PANL 7 8 DISPLAY CLEAR PROD RA PL01 IFB4 TD AD NEWS LISTEN T1 TEL1 CALL TEL2 CLR FLOR MULT 0 FUNC CHYR AUTO 3 ISO 6 9 PGM ISO1 ISO2 AUD1 CLEAR CALL NUM 1 SUST PHONE 4 RELAY COPY 7 CLR PL 2 IFB 5 E-PANL 8 DISPLAY MULT 0 FUNC AUTO 3 ISO 6 9 PGM Figure 1.5 Complex Matrix Intercom System MATRIX Audio IN, Audio OUT, Data Analog Audio Third Party Terminal Equipment Keypanel LAN / WAN Third Party Terminal Equipment System Audio IN, Audio OUT, Data News Computers Third Party Terminal Equipment Keypanel Before We Begin In many cases, connection to the telephone company is required to allow a reporter to connect into an intercom from his or her cell phone, or to allow a return program feed to be fed to a remote location. A telephone interface (TIF) unit provides this connectivity. The most basic accessory in an Intercom system may be the headset. It may provide isolation from ambient noise; it may have a noise-canceling microphone to reduce wind noise, and may have stereo ear pieces to allow program audio and intercom audio to be fed independently to the right and left ears. Each of these accessories is vital to creating an intercom system that meets the communications needs of the users. Throughout this book, you will be subjected to the jargon that permeates the intercom world. In the chapters that follow, you will be presented with definitions specific to the topic being covered. In many cases, there are common terms that will be applicable to all these chapters, and so we will present a few definitions to get us started. We have also provided a comprehensive glossary in the rear of the book. IFB Interrupted Fold Back also referred to as IRF Interrupted Return Feed. The best way to explain this is to give an example. A news reporter is on the scene of live accident coverage. She needs to not only hear what the anchor back at the studio is saying i.e., So, Jane, how many chickens were injured when they tried to cross the road during rush hour? She also needs to hear instructions from the director back in the studio i.e., Wrap it up, 10 seconds. The IFB function in an intercom system allows a single audio signal to be sent to Jane, normally containing program audio interrupted by instructions or information from someone not a part of the program audio. Chapter 1 - Intercoms An Overview 5

20 ISO Camera Isolate This is not reserved strictly for the domain of cameras anymore. This is truly an isolate function, not unlike the action at a party of grabbing the arm of a fellow guest, dragging them off to a corner for a private conversation, and then returning them to their group. There are instances where it is necessary in an intercom system to establish a momentary private conversation with someone who may be talking and listening to a number of other people. The person who needs to interrupt presses a button or key, which establishes a private two person conversation. Upon releasing the key, the two participants are returned to whatever conversation(s) they were a part of previously. This was called Camera Isolate as it first was used to remove an individual camera from a conference to allow private communications. Tally A signal sent for the purpose of indicating status for a particular purpose. The sound of your telephone ringing can be described as a tally. On an intercom panel with multiple channels, it can be a visual signal, such as a blinking light, to indicate which station is calling. It can be used to indicate a particular function is not available due to a conflict similar to the busy signal you get when calling the radio station trying to be the tenth caller and win a year s supply of cat litter. The above definitions and many more can be found in the glossary at the back of this book. The Rest Of The Book We have organized this book by the above types of systems two chapters devoted to PL Intercoms, two chapters for matrix systems, two chapters for wireless systems, and one chapter on interfaces, determining systems needs and requirements, technical requirements for installation, and some real world case studies. Near the end of the book, we have included references for further information, a glossary, and a CD full of information on Telex Products, technical references, and many system drawings. 6 Handbook of Intercom Systems Engineering

21 C HAPTER 2 2INTRODUCTION TO PARTY-LINE CHAPTER INTERCOM SYSTEMS STAN HUBLER Introduction Some Definitions Leading off this chapter, Some Definitions that may help you understand Party-Line intercoms terms (and buzz-words). Then, a Short History of Party-Line intercoms will be presented, leading into a discussion of Present Day Systems and Manufacturers. The System Components and Their Function will explore the main components of these systems and what they do. Then, How Each System Works shows how these system components are put together to make a functioning intercom and some examples of the different systems. Outstanding Features of Each System describes application areas and where each system is often marketed. Some important Limitations of Each System are described and a Summary closes this chapter. Party-Line (PL) systems / Conference Line Intercom Systems A Party-Line system allows a group of people to intercommunicate. For example, one person can talk, while all the others on the bus or channel can hear. When the system is full duplex, anyone can talk and the rest can hear or interrupt the speaker at any time. The Party-Line and distributed matrix systems presently sold today are usually full duplex and are non-blocking, which means that access to the channel is immediate and there is no busy signal. Conversations on Party-Line systems are, in general, non-private. It is important to note that both two wire and four wire type systems support the Party-Line concept. Two-Wire A communications system where the path is the same for both talk and listen. In electrical pathways there are, in fact, two wires (one path). Two-wire systems can be two-wire balanced or two-wire unbalanced. Chapter 2 - Introduction to Party-Line Intercom Systems 7

22 Balanced Line The balanced line concept reduces noise pickup by outside sources. A balanced two conductor line carries audio that is differentially driven and balanced to ground. Full Duplex This is communication that allows simultaneous two-way conversations, that is, one person can interrupt the other. In data communications, full duplex permits confirmation of sent data by the receiving terminal echoing, sending back the same data, or confirming data. Decibel (db) A derived unit of loudness. The human ear perceives a 10 decibel increase as twice as loud, and a 10 decibel decrease as half as loud. Beltpack A portable headset user station. This station is designed to be worn on a user s belt, but is also fastened to the underside of consoles, taped to a structure near the user, or mounted on a piece of equipment. The headset plugs into the user station, as does the connection to the rest of the intercom. Biscuit Marketing buzz word for a portable speaker station. Main Station A multichannel user station. There may be one or more of these stations in a system. Usually the primary station in a system. Master Station A user station where a user station and a system power supply are combined into one package Sidetone In the truest sense, sidetone is a small amount of microphone signal fed back to the earphone of the individual speaking into the microphone. In a two type user station, the null balance control is sometimes used to adjust the amount of sidetone the user hears. This control is sometimes (erroneously) called the sidetone control. Other equipment has both null balance adjustments and a true sidetone adjustment. Crosstalk A Short History Unwanted interference caused by audio energy from one line coupling ( leaking ) into adjacent or nearby lines. Party-Line intercoms were needed early on by television production crews to coordinate their activities. Some of the activities included on-site sport pickups, entertainment on stage, and videotaping of shows. The crews included camera operators, audio, lighting, 8 Handbook of Intercom Systems Engineering

23 stage directors, director, assistant director, production assistant, and others. Originally, these crews shared one intercom channel where the director called the shots. Later, as intercom developed, additional channels were added so each crew could still listen to the director, then could switch to their own channels to coordinate activities without conflict with the director. Party-Line intercom systems were also used by industrial activities to coordinate manufacturing and testing of large systems such as aircraft. Early intercom systems ( ) were either homemade or accumulations of telephone equipment lashed together. Often, the homemade intercoms worked well enough but lacked the flexibility to expand the system or interface with other systems. The telephone equipment approach had some flexibility, but performance degraded rapidly as the number of stations increased above ten user stations. In the early 1970s, Clear-Com built Party-Line systems for rock-n-roll concerts, and later for theatrical stage, and eventually for television production. This system was flexible and expandable, but required one three-conductor microphone cable for each channel. In the mid 1970s, another company, RTS Systems, designed a system for television production that had two channels on one three-conductor microphone cable (or one channel on a pair of wires). This system was even more flexible and expandable with a design that allowed up to 50 user stations on a single channel. On the East Coast, a company, Chaos, produced intercoms for the New York and other stages. And, in the Midwest, a company, Telex Communications, produced a balanced Party-Line system. This system was especially useful in noisy electrical environments, because it was immune to induced interference. Other Party-Line systems include systems such as David Clark, which is used for fire trucks and similar public safety and service crews. And, of course, four wire matrix systems can emulate Party-Line intercoms. As Clear-Com and RTS Systems intercoms became more widely known, compatible systems of both appeared. They included HME and Production Intercoms for Clear-Com, and ROH and Anchor Audio s PortaCom for RTS. Chaos is similar to Clear-Com, except it uses a much higher power supply voltage (46 vs. 24 volts). As the markets expanded, the distinction between theatrical and television production became blurred and Party-Line systems of all types were used wherever they were needed. So a competitive atmosphere developed and continues to the present. ROH and HME are no longer in the wired intercom market. Present Day Systems and Manufacturers The three major brands of two-wire Party-Line intercoms having the largest worldwide presence are RTS, Clear-Com, and Telex Audiocom. Other brands include Chaos, David Clark, PortaCom, and Production Intercom. Table 2.1 Intercom brand name vs. manufacturer. Brand Name Audiocom Chaos Clear-Com David Clark PortaCom Production Intercom RTS Manufacturer Telex Communications, Inc. Goddard Design Company Clear-Com Intercom Systems David Clark Company, Inc. Anchor Audio, Inc. Production Intercom, Inc. Telex Communications, Inc. Note Present day Party-Line intercom systems are mostly distributed amplifier type systems as opposed to a centralized system where all the headset lines plug into one box (Some David Clark Systems are of a centralized type). Oh yes, there is a no-amplifier system called a Chapter 2 - Introduction to Party-Line Intercom Systems 9